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Animal Psychoacoustics
ers must estimate the distance to a sender to know whether an intruder is located within or outside their territory or to be able to locate a potential mate.
Ambient (or environmental) noise is a particularly interest- ing problem in acoustic communication by animals (e.g., Wiley, 2015). Whether it comes from other animals, weather, or humans, it is clear that at least some animals adjust their communications based on the levels and spectral features of the ambient noise when they emit signals (e.g., Brown et al., 1995). Several field studies have shown that animals have ad- justed their communication signals over the past few years or decades to compensate for either anthropogenic or other noise sources (reviewed in Wiley, 2015). From many studies, we can see the clear influence that humans are having on the lives of animals (e.g., Wiley, 2015). It would be of great importance, therefore, to know as much as we possibly can about what animals can and cannot hear, how noise affects their ability to communicate, and exactly how detrimental environmental disruptions of communication can be to an animal’s life. Very important, results from such animal bio- acoustics studies are used to implement environmental stan- dards for noise and human involvement in animal popula- tions.
Thus, to survive and thrive, animals need to be able to de- tect, discriminate, localize, identify, and categorize all types of sounds to navigate their complex acoustic world effec- tively (Bradbury and Vehrencamp, 2011). To determine an animal’s perception of the world, it is first important to un- derstand simple hearing abilities under controlled laborato- ry conditions before one can begin to speculate about their communication strategies and abilities under more natural settings using more natural stimuli.
Psychophysics is a method of presenting stimuli to an organ- ism to “determine the limits and dimensions of its sensory experience” (Stebbins, 1970). Psychoacoustics is a subfield of psychophysics where the stimuli are presented in the au- ditory domain. In animals, psychoacoustic experiments are often lengthy and difficult behavioral experiments to con- duct, but they are considered the “gold standard” by most researchers in the field of audition (Klump et al., 1995). Reliable, trained, awake, behaving animals are telling the researcher how they perceive the world: whether a sound came from the left or the right, whether a sound was from a partner or a stranger, or what exactly the signal was that was obscured by noise. Psychoacoustic experiments have been
used for many years to answer questions such as these (Fay, 1988), and the more data obtained from such experiments, the better understanding we have of the communication be- havior of a given species.
Psychoacoustics has been used to determine an animal’s sensory-processing acuity for decades (e.g., Stebbins, 1970). Early studies on an animal species typically vary acoustic stimuli along one dimension (e.g., frequency, duration, in- tensity) at first and then move on to more complex, natural- istic stimuli varying in multiple dimensions (Fay, 1992). Just noticeable differences or the smallest change in an object that can be noticed about 50% of the time can be determined for both barely detectable and much louder stimuli. Typi- cally, signal detection theory techniques are employed to calculate thresholds (Green and Swets, 1966). Stimuli with changes the animals can detect and changes they cannot de- tect are presented to the animals for numerous trials, and the rate of “hits” is compared with the rate of “misses.” On other trials, no change occurs in the stimulus presentation and the animal is typically required to withhold respond- ing for a “correct reject.” If they respond on those no-change trials, a “false alarm” is recorded. Hits, misses, false alarms, and correct reject rates are all used to determine whether the animals are under stimulus control and actually responding to the experimental task in the way they were trained to re- spond. Importantly, these are also used to determine thresh- olds for each experimental condition by separating response biases the animals may have (animals, like humans, may be more or less conservative) from their sensitivity.
The various psychophysical methods used to determine auditory acuity in humans (Zwicker and Fastl, 1999) have various levels of success when used with animal subjects (summarized in Klump et al., 1995). In humans, popular psychophysical approaches include the method of constant stimuli, where a set of stimuli is presented both above and below the estimated threshold randomly for many hundreds or thousands of trials. Subjects respond “yes, I hear it” or “no, I do not hear it,” and then the thresholds are determined from an average response of the many repetitions of those trials.
Adaptive tracking procedures, where a sound characteristic (e.g., intensity) is incrementally lowered until a subject no longer reports hearing it and then is raised and lowered sev- eral more times until a mean threshold value of the reversals is obtained, are also widely used in humans and animals.
20 | Acoustics Today | Fall 2017